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S E C T I O N 37. 2 • Young’s Double-Slit Experiment 1181 ▲ PITFALL PREVENTION 37.3 It May Not Be True That L !! d Equations 37.2, 37.3, 37.5, and ▲ PITFALL PREVENTION 37.2 It May Not Be True That ! Is Small The approximation sin! ! tan! is true to three-digit precision A viewing screen is separated from a double-slit source by (A) Determine the wavelength of the light. Solution We can use Equation 37.5, with m # 2, y bright # 4.5 * 10 $ 2 m, L # 1.2 m, and d # 3.0 * 10 $ 5 m: which is in the green range of visible light. 560 nm # 5.6 * 10 $ 7 m # & # y bright d mL # (4.5 * 10 $ 2 m)(3.0 * 10 $ 5 m) 2(1.2 m) (B) Calculate the distance between adjacent bright fringes. Solution From Equation 37.5 and the results of part (A), 2.2 cm # 2.2 * 10 $ 2 m # # & L d # (5.6 * 10 $ 7 m)(1.2 m) 3.0 * 10 $ 5 m y m(1 $ y m # & L d
(m ( 1) $ & L d
m Example 37.1 Measuring the Wavelength of a Light Source Interactive we see that y # L tan! ! L sin! (37.4) Solving Equation 37.2 for sin! and substituting the result into Equation 37.4, we (37.5) Using Equations 37.3 and 37.4, we find that the dark fringes are located at (37.6) As we demonstrate in Example 37.1, Young’s double-slit experiment provides a method for measuring the wavelength of light. In fact, Young used this technique to do y
dark # & L d (m ( 1 2 )
(m # 0, %1, %2,
) ) )) y
bright # & L d m
(m # 0, %1, %2,
) ) )) Quick Quiz 37.1 If you were to blow smoke into the space between the barrier and the viewing screen of Figure 37.5a, the smoke would show (a) no evidence Quick Quiz 37.2 In a two-slit interference pattern projected on a screen, the fringes are equally spaced on the screen (a) everywhere (b) only for large angles Quick Quiz 37.3 Which of the following will cause the fringes in a two-slit interference pattern to move farther apart? (a) decreasing the wavelength of the light Investigate the double-slit interference pattern at the Interactive Worked Example link at http://www.pse6.com. |